Electric machines typically comprise a stator element and a rotor element that interact electro-magnetically to convert electric power to mechanical power or to convert mechanical power to electrical power. For example, a conventional stator element comprises an annular housing having windings of copper coils circumferentially oriented. A conventional rotor element is mounted on a shaft for rotation. Electric current is passed through the stator windings to generate an electro-magnetic field that causes the rotor and shaft to rotate about an axis of rotation of the shaft. The electric current causes resistance heating of the coils, which heats the entire electric machine including the rotor. In particular, high power electric motors that operate at high speeds and are compact in size generate high heat densities.
Conventional schemes for cooling electric machines involve passing cooling fluid over the stator, which is typically easy to accomplish because of the stationary and exterior nature of the stator assembly. Rotors are typically cooled by passing cooling fluid between the stator and rotor. Such cooling schemes, however, often provide inadequate cooling for rotors used in high power motors due to high rotor heat density. In addition, as the size of high power motors decreases, the available area for passing cooing air between the stator and rotor also decreases. There is therefore a need to improve cooling efficiency in electric machines.